Method of forming an electrostatic printing base



Oct. 17, 1967 J. J. CLANCY METHOD OF FORMING AN ELECTROSTATIC PRINTING BASE Original Filed April 16, 1963 PHOTOCONDUCTIVE PARTICLES COATING FILM BINDER 1 n, COATING //'SUBSTRATEV 20' I 1 lo GROUNDED METAL CONDUCTOR 'Fig.4

INVENTOR.

John J. Clancy w 4 WI Attorney United States Patent 3,347,702 METHOD OF FORMING AN ELECTROSTATIC PRINTING BASE John J. Clancy, Westwood, Mass., assignor to Arthur D. Little Inc., Cambridge, Mass., a corporation of Massachusetts Original application Apr. 16, 1963, Ser. No. 273,404. Divided and this application Sept. 12, 1966, Ser. No. 594,300

Claims. (Cl. 117201) This application is a divisional of my copending application Serial No. 273,404, filed April 16, 1963.

This invention relates to electrophotographic reproduction and more particularly to an improved coating for substrates to be used in the process of electrostatic printmg.

Electrostatic printing is a process for producing a visible record, a reproduction or a copy which includes as an intermediate step the conversion of a light image or image signals to an electrostatic charge pattern or image on a printing base. In the case of electrophotographic processes an electrostatic latent image or an electron latent image is produced on a charged surface by utilizing the property of photoconductivity (i.e., a variable conductivity depending upon the intensity of illumination which strikes the coating). As will become apparent in the following description the electrostatic latent image may be produced in a conventional exposure operation such as, for example, through a negative, or it may be created through the use of mirrors or other optical devices. The latent image thus created may be developed directly by toning with a charged powder, liquid or aerosol; or the image may be toned and the toned image transferred to a second substrate where the image is fixed.

In electrophotographic processes the electrostatic latent image is commonly formed on the surface of a photoconductive dielectric layer carried on a supporting substrate. Typically, materials comprising a substrate and a photoconductive layer thereon are sensitized by applying a uniform surface charge to the free surface of the photoconductive layer. Such surface charge may be applied for example by means of a corona discharge, which charge is retained owing to the substantial insulating character, that is, the low conductivity of the layer when not exposed to light. On exposure to light, the photoconductive property of the layer causes the conductivity to increase in the illuminated areas, the extent of conductivity being dependent upon the intensity of the illumination. The surface charge in the illuminated areas leaks away leaving the charge located in the unilluminated or unexposed areas where it is desired to produce indicia. This remaining charge constitutes the charged pattern or electrostatic latent image, which then may be developed and used directly or transferred to a second substrate.

Such electrophotographic reproduction processes have found wide acceptance in recent years, particularly in the field of office copying. They possess many inherent advantages, among which may be listed the elimination of liquid developing agents, the elimination of any intermediate step using a photosensitive plate if the originally charged materials are developed directly, and the ability to achieve continuous tone images.

Normally in the process of electrostatic printing a nonconductive substrate such as paper, or a conductive substrate such as aluminum foil, is coated with a material which contains a finely divided photosensitive material. Such photosensitive or photoconductive material is normally zinc oxide, although other materials such as lead iodide, arsenic trisulfide, or cadmium selenide may be used. These particles being photosensitive are capable of electrical conduction upon exposure to light. As an alternative to using photoconductive particles suspended in a binder material, it has also been found possible to make a photoconductive film of certain film-forming materials.

Typically, such a film-forming material is poly-(N-vinyl carbazole). However, a number of other organic photoconductors have been disclosed. (See for example US. Patent 3,041,165.)

In the use of the photoconductive particles suspended in a binder, it is frequently desirable to restrict the choice of binders to certain specified materials, and to add to the photoconductive coating a dye capable of enlarging the absorption band of the coating material. Moreover, in order to obtain a satisfactory coating which can be used in the manner described it has always been necessary to use relatively heavy coating weights, i.e., of the order of about 30 pounds of coating per ream (3,000 square feet) of paper or other substrate. Considering that paper used for the purpose is typically 51 pound paper (3,000 square feet ream basis), it will be seen that the finished photoconductive coated paper weighs 81 pounds-a relatively heavy paper by modern standards. Thus, if it were possible to make an equally efiicient photoconductive coating which was considerably lighter than that now used, the resulting savings in weight handling alone would be material.

I have found that much lighter weight photoconductive coatings can be made by forming the binder materials into films which contain a large proportion of air voids. Not only does this film structure reduce weight but, unexpectedly, it provides a markedly more eflicient photoconductive material, being more sensitive to a fixed light flux and hence a faster film when used in the electrostatic reproduction process described above.

It is, therefore, a primary object of this invention to provide a novel type of coating suitable for application to a substrate, the finished coated substrate to be used in an electrophotographic or electrostatic printing process. It is another object of this invention to provide a coating of the character described which is materially lighter in weight and more sensitive to light than the presently available coatings. It is yet another object of this invention to provide such a coating which may be formed using many binders which are not usable in the present type of photosensitive compositions. It is yet another object of this invention to provide photosensitive compositions, suitable for coating on a substrate, which are capable of exhibiting improved water resistance, and of being applied as a papermaking step, thus saving an additional converting step. Another primary object of this invention is to provide an improved coated substrate suitable for use in electrostatic copying methods. Another object is to provide a coated substrate of the character described which is lighter in weight and which is more sensitive to light and less sensitive to moisture than those now in use. Other objectives of the invention will, in part, be obvious and will, in part, be apparent hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the article possessing the features, properties, and the relation of elements which are exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

The photoconductive coating of this invention may be described as a binder in the form of an essentially continuous film containing multitudinous air voids uniformly distributed throughout, the air voids varying in maximum dimension between about 0.5 micron and 10 microns. The film, without photoconductive particulate matter, may be further characterized as having a density of between about 0.1 gm./cc. and 0.6 gm./cc. depending at least partly on the binder used. If the binder is a dielectric material which is not itself photoconductive in nature, the film will also contain finely divided photoconductive particle uniformly distributed throughout the essentially continuous binder J film matrix. The structure of the finished coating of this invention may be further characterized as having a vastly increased surface area by reason of the voids. Because of its structure, a coating made in accordance with the teaching of this invention in a thickness equivalent to the thickness of prior art solid film coatings will weigh only about 15% to 30% that of the solid film coating. Applying this to a practical example, it will be seen that instead of converting l-pound per ream paper (3,000 square feet) to 81-pound per ream paper to make it a photosensitive sheet the coating of this invention converts the same 51-pound per ream paper to 60 to 63 pound paper to achieve the same or improved reproduction. Because lighter weight coatings are used, this invention anticipates the use of a lighter weight substrate, thus giving an appreciable reduction in the total weight of the product.

The voids in the coating may be obtained by several methods as will become apparent from the description be-. low. They may, however, be generally described as having been created through the formation of partially open air-binder interfaces, or through the formation of essentially enclosed voids. The film formed with partially open air-binder interfaces exhibits a high orderof opaqueness and brightness due to the structure of the film which achieves good light scattering.

The voids in the finished photosensitive coating must vary within a specified size range as defined below in order to be most effective, to balance the optical and electrical properties of the finished coating, as well as .to achieve acceptable strengthin the coating itself.

In t-he coating of a substrate to form the photosensitive material of this invention it is preferable to apply the original liquid coating composition in the form of an emul-,

sion, the discontinuous and continuous phases being formed of immiscible liquids which exhibit adiiference in vapor pressure at the temperature at which drying of the coating takes place. Coatings which are applied as emulsion films are dried in a manner to retain the original emulsion structure in the dried film, thus the emulsion is not broken and the film-forming binder of the coating is never converted to finely divided particles, but remains in the form of a continuous film containing the necessary voids uniformly distributed throughout.

Suitable coating emulsions may be the oil-in-water type or the water-in-oil type, depending upon whether the film binder material is soluble in water or oil. If the film binder material is soluble in water, then the emulsion will be of the oil-in-water type; while, on the other hand, if the filin binder material is water insoluble, it will be of the water-in-oil type. If'the finely divided particulate matter, such as zinc oxide, is used in the coating to give it its photosensitive properties, then this finely divided photoconductive material is preferably introduced into that phase of the emulsion which becomes the continuous film of the finshed coating, that is, it will be introduced into the continuous phase whether it is the oil-soluble or water-s0lublephase. Inasmuch as organic polymeric binder materials which in themselves are photosensitive are not normally water soluble,.they will be made into .water-in-oil emulsions, the binder material being dissolved in a suitable organic. water-immiscible liquid.

Techniques other than the use of emulsion coatings for introducing the required voids into the finished photoconductive films are not precluded. Thus, the film coating may be deposited as a foam, or as a liquid containinga volatile blowing agent. The primary requirement for any voidcontaining film forming technique is that the voids formed are within the size range specified.

For a fullerunderstanding of the nature and objects of the invention reference should be had to the following detailed description taken in connection with the accompanying drawings in which:

FIGS. 1 and 2 are fragmentary cross-sectional views of a substrate carrying the photoconductive coating of this invention containing a finely divided photoconductive material, such as zinc oxide;

FIG. 3 is a fragmentary cross-sectional view of a substrate carrying a photoconductive film of this invention made from a photoconductive organic film-forming material;

FIG. 4 is a cross-sectional view showing the manner in which a photoconducting paper is charged;

FIG. 5 illustrates the step of exposing a charged photosensitive paper to a master negative to form a latent electrostatic image;

FIG. 6 illustrates the step of toning the exposed paper of FIG. 4; and

FIG. 7 illustratesthe transfer of a toned image to a second substrate.

The structure of the photoconductive film of this invention is shown in diagrammatic fashion (much enlarged and not to scale) in FIGS. 1-3. FIGS. 1 and 2 show a coating which is made photoconductive by virtue of presence of finely divided particulate matter within the continuous binder film, zinc oxide being used for illustration only. In FIG. 1 the substrate 10, which may be an electrically nonconductive material such as paper or an electrically conducting material such as a metal foil, carries on its surface the coating 11 which in this case is seen to be formed of a continuous binder material 12 having distributed uniformly throughout it finely divided particulate matter 13 which'in this example is photoconductive zinc oxide. Also distributed throughout the continuous film binder matrix 12 are a multiplicity of air binder interfaces 14. FIG. 2 illustrates an alternative coating which is formed in such a way that the voids14a are essentially enclosed rather than in the form of the partially broken air-binder interfaces 14 of FIG. 1. In general, emulsion coatings of the oil-in-water, type will formfilms like that illustrated in FIG. 1; while emulsion coatings of the waterin-oil type will form films like that illustrated in FIG.2.

FIG. 3 illustrates another embodiment of the photoconductive material of this invention. This film is formed of an organic film-forming material which in itself is photoconductive. In this case the coating 11 is seen to consist of the continuous film 15 having distributed t-hroughoutthe air-matrix interfaces 14 throughout its entire volume. These air-matrix interfaces make up the necessary air voids in the finished coating. 11.

FIG. 4 illustrates the electrical charging of the photoconductive materials of FIGS. 1-3 for further processing.

Conveniently this is done by locating the coated substrate on a grounded metal conductor 18 and placing above it a series of fine wires 19 (only one of which is shown in the cross-section of FIG. 4). When a high DC voltage of the order of 5,000-6,000 volts is applied across the wire and the grounded metal conductor, the coating 11 is charged on its surface. It is then ready to be exposed to a photographic negative or to other master copy in such a way that the light striking the charged coating will vary over the surface corresponding to the intensity of the image to be reproduced. As illustrated in FIG. 5, light from a source 23 is directed through a master copy 24 which is to be reproduced. In this case the letter A designated by nu- I meral 26 is the indicia which is to be duplicated on the photosensitive paper 16, positioned below the master 24. It will be appreciated that considerably more light passes through the lighter area around the letter A than through the letter A. Thus, there is created on the photosensitive paper 16 an indicia corresponding to letter A in the form of an electrostatic image thereon. Where light has been permitted to pass through and strike the photosensitive paper 16 the charge is dissipated and leaks away. However, where it does not strike the photosensitive paper 16 (i.e., in that area defining the letter A) there remains an electrostatic charge on the surface of the paper. This electrostatic image may then be stored for a time but normally it will be developed to render it visible or to render it in a form which can be transferred to a second substrate.

Development normally takes the form of applying a toner to the surface 27; the toner being a material which bears the opposite electrostatic charge from that of the electrostatic image. Hence, it is attracted and held to the image such as shown in FIG. 6'. With the application of a toning material 29 from a source 30 there is formed upon the sheet 16 a visible image 28. The toner material may be in the form of a powder, a liquid or an aerosol in keeping with Well-developed techniques. New toning techniques are not a part of this invention and any toner which is usable in this process may be used in the method of this invention.

Once the visible image has been established on the sheet 16 is may be fixed to it such as by heating or by the use of a solvent which will cause the toning material to permanently adhere to the substrate. Alternatively, the toned, but unfixed, image as illustrated in FIG. 6 may be used to transfer to another substrate where it is fixed as permanent copy. The transferring of the toned image to a second substrate is normally carried out by using an electrically conducting plate of opposite charge to the toner used and placing it under the substrate to which the toned image is to be transferred. The first substrate may then be used again. This step is diagrammatically illustrated in FIG. 7. A second substrate 32 is placed in contact with the first substrate 16 hearing the toned and electrically charged image 28. (It will be appreciated that virtual contact between the substrates 16 and 32 is required and that the separation between them in FIG. 7 is made for purposes of ease of illustration only.) An electrically charged plate 33 deriving a charge from a source 34 is placed in contact with the surface of the substrate 32 Which is not in contact with the toned image. The charge on the plate 33 must, of course, be opposite in polarity to the toned image to attract the toner to substrate 32 which may be of any material suitable to permanently aflix the toned image thereto.

In addition to the procedures which produce a latent electrostatic image and then develop this image as illustrated in FIGS. 4-7, it is possible to form a latent electron image, or latent conductivity image on the photoconductive film. This process differs from that described above in that a dark adapted photoconductive layer is first exposed to light as shown in FIG. 5, thus exciting electrons from the valence band into shallow traps lying near the conduction band. This latent image is developed by sprinkling toner over the surface and then applying a potential between the toner and the ground plate on which the photoconductive sheet lies. When the toner is positive and the ground plate negative excited electrons are extracted from the portion of the layer that was previously exposed to light, and these electrons neutralize the toner which subsequently falls otf. The areas that were not previously exposed to light are incapable of neutralizing the toner, which is, therefore, electrostatically bound to the photoconductive sheet. The image can then either be transferred'to another sheet or fixed as described above.

It will be seen from the above description of the process of electrostatic'printing that the coating must possess certain characteristics in order to make it an efiective product. It must, of course, be photosensitive and it must also be capable of holding a charge deposited on its surface. Further, it is highly desirable that such a coating be moisture-insensitive to make it operable under essentially all conditions of humidity without any marked variations in its performance. Such a coating should also be insensitive to pressure to be encountered in its normal handling. Moreover, it should respond to a relatively wide spectral hand. If the coated substrate is to be used as the final copy, then it should also be a coating which in itself offers a pleasing background as measured in terms of opaqueness and brightness. Finally, it should be lightweight and inexpensive.

Many of the photosensitive particulatev materials, e.g., zinc oxide, which are suitable for photoconductive papers and the like are, unfortunately, sensitive to a very narrow wavelength band in the spectrum normally used in exposing them. In keeping with well developed techniques, this invention contemplates the addition of certain dyes to the photoconductive coating to widen the spectral band to which it may be responsive, if such dyes are needed. Each photoconductive material has a peak response characteristic of its particular composition and is influenced by such ingredients asthe binder used in the coating. For example the peak response of zinc oxide is 3750 A., of thallium iodide about 4130 A. and of silver sulfide about 13,500 A. The dyes which may be added will of course be chosen to complement and broaden the spectral response which includes the peak response region of the photoconductive material. For example, xanthane, thiazole, thiazine, and diphenylmethane dyes have been found to be particularly Well suited dyes for use with zinc oxide, and conventional binders.

Typical dyes which are suitable for incorporation into photoconductive coatings to broaden their spectral responses include the xanthane dyes such as Uranine (CI Acid Yellow 73), Eosine (CI Acid Red 87) and Rose Bengal (CI Acid Red 74); the triarylmethane dyes as Crystal Violet (CI Basic Violet 3), Brilliant Green (CI Basic Green 1) and Patent Blue (CI Acid Blue 9) the thiazol dyes such as Thioflavine TG (CI Basic Yellow 1); the thiazine dyes such as Methylene Green (CI Basic Green 5 and Methylene Blue (CI Basic Blue 9); the azine dyes such as Methylene Violet (CI Basic Violet 5); the acridine dyes such as Acridine Orange (CI Basic Orange 14); the diphenylmethane dyes such as Auramine 0 (CI Basic Yellow 2); the cyanine dyes such as Thiazole Purple (3,3'-diethylthiacarbocyanine iodide); anthraquinone dyes such as Alizarine Red (CI Mordant Red 3); and mixtures of dyes. such as Methylene Grey (CI Basic Black 1).

The following examples which are meant to be illustrative and not limiting are given to further describe the method and product of this invention.

Example I An oil-in-water emulsion was made using casein dispersed in water as a continuous phase and a water immiscible solvent such as kerosene as the discontinuous phase. 415 grams of a 12% casein dispersion in water was mixed with 300 grams of zinc oxide (photoconductive grade) and 50 grams of water. The zinc oxide used was made by the French process and is sold as Florence Green-Seal zinc oxide by New Jersey Zinc Company.

The ratio of zinc oxide to casein in this mixture was 6 to 1. To 200 grams of the casein-zinc oxide mixture was added 3 grams of ammonium oleate and this was rapidly stirred while 78.5 grams of kerosene was added to form an oil-in-water emulsion. In order to provide a control sample of the film which did not contain the voids in its final form 200 grams of the same casein-zinc oxide-water mixture was used containing 3 grams of ammonium oleate. Both of the coatings were applied to a blank paper so that there was a 4 mil wet coating on each of them. The samples were dried by heating in an air oven maintained at 260 F. for about 2 minutes. The coating applied as an oil-in-water emulsion under these conditions dried to form an essentially uncollapsed continuous film having multitudinous air-casein interfaces distributed throughout. In drying, a portion of the water was first removed through volatilization transforming the casein into a gel-like continuous film matrix having the kerosene droplets suspended in it in essentially the same relation ship and size as they existed in the emulsion film coating. Further drying volatilized the kerosene leaving the aircasein interfaces where the kerosene was removed. Coatings made in this manner are extremely bright and opaque.

charged as by the procedure described in accordance with the description of FIG. 4 using a corona discharge maintained at 5,000 volts. Each of the samples was then exposed to light using a standard step tablet designed for evaluating sensitivity of a material to decreasing intensities'of light. The step tablet used had 21 steps with each succeeding step being about 0.15 density value darker. Diffuse density is a measure of the amount of incident light transmitted through the Step wedge. It is a log function expressed as log 1/ T where T is light transmission expressed as a decimal.

The step tablet was placed in turn over each of the charged paper samples which had been prepared as described above. Each sample was then exposed through the step tablet to light-from an Omega Enlarger (type B6 with a 75-wattPH-11A lamp) for 15 seconds. The samples were then toned to produce a visible image of the step tablet. The sensitivity (efficiency) of photoconductive films is rated by determining the first step of lighter density than the undischarged portion of the sample; this step indicates the minimum amount of light to which'the sample is sensitive. In the case of the coating put on in the form of an emulsion and containing voids as required in this invention an intensity difference was discernible between steps 6 and 7, while in the case of the solid casein film coating put on as a control the intensity difference was noted between steps 4 and 5. This is a significant difference inasmuch as each step represents a marked improvement in sensitivity.

Example 2 This example is directed to the use of a dilferent type of binder material, namely polyvinyl alcohol. Since this binder is also soluble in water, it was made up as an oilin-water emulsion, the polyvinyl-alcohol solution being the continuous phase and forming the film which contained the zinc oxide photoconductive material.

50 grams of polyvinyl alcohol was dispersed in 363 grams of water by known techniques. To this was added 300 grams of the photoconductive grade of zinc oxide of Example 1. This gave a mixture which was 42% by weight zinc oxide. 3.5 grams of ammonium oleate was added to 200 grams of the master polyvinyl alcohol-zinc oxide dispersion, and then with rapid stirring 84 grams of No. 9 solvent (a hydrocarbon having an initial boiling point of 335 F. and a boiling range from 335 to 515 F.) was added to form an oil-in-water emulsion. As a control, 3.5 grams of ammonium oleate was added to 200 grams of the master polyvinyl alcohol-zinc oxide dispersion. The two coatings were then applied to paper as 4 mil wet thickness coatings. Drying was accomplished as in Example 1 andthe samples were conditioned at 50% relative humidity. The coated samples were charged as in Example 1 and then exposed to the step tablet as described. In the coating which contained the voids and which was formed from the emulsion, the intensity difference was found between steps 5 and 6, while in the control the intensity difference occurred between steps 3 and 4.

Example 3 This example is directed to the preparation of an emulsion of the water-in-oil type. The binder used was. ethyl cellulose which is soluble in benzene but insoluble in water. The master coating Was formed by dissolving 41 grams of ethyl cellulose in 650 cc. of benzene. To this was added 1.9 grams of Triton X-400 (an emulsifier formed of stearyl dimethyl benzyl ammonium chloride and related cationics) and 206 grams of zinc oxide. The

approximately 4 mils wet thickness. They were dried, conditioned, and charged with corona discharge of 5,000 volts as in Example 1. The charged coatings were then exposed to the step tablet standard and examined for determination of the location of intensity difference. In the case where the coating had been deposited as a filmcoating with voids, the intensity difference was noted between steps 5 and 6. In the control coating which was applied as a solid film no intensity difference was noted whatsoever.

Example 4 This example is directed to the use of a dye to broaden the spectral response of a photoconductive coating containing air voids. Fifty grams of a 12% casein dispersion was mixed with 108 grams of zinc oxide (photoconductive grade as used in Example 1), 33.4 grams of water, and 0.22 gram of Uranine (CI Acid Yellow 73). An oilin-water emulsion, using casein dispersed in water as the continuous phase, was prepared by stirring 36 grams of No. 9 solvent into a mixture of 50 grams of a 12% casein dispersion, 1.4 grams of oleic acid, 2.2 grams of a 28% solution of ammonia in water, and 10.4 grams of water; This emulsion was then added to 95.7 grams of the casein-zinc oxide-Uranine mixture prepared as above. To provide a control sample which did not contain air voids, 50 grams of a 12% casein dispersion was added to 95.7 grams of the casein-zinc oxide-Uranine mixture prepared as above. The two coatings were. then applied to paper as 4 mil wet thickness coatings. Drying was accomplished as in Example 1, and the samples were conditionedat 50% relative humidity. The coated samples were charged as in Example 1 and then exposed to the step tablet. as. described. 'In the coating which contained the voids and which was formed from the emulsion, the

intensity difference was noted between steps .6 and 7;

while in the control the intensity difference occurred between steps 3 and 4.

Example 5 Thisexarnple is directed to showing the effect of the size of the air voids on the photoconductive properties of Microns Unhomogenized 2-4 Homogenized at 1,000 psi 0.5-2 Homogenized at 2,500 p.s.i 0.5-1 Homogenized at 4,000 p.s.i. 0.5

grams of a 12% casein dispersion was mixed with 216 grams of zinc oxide (photoconductive grade of Example 1) and 66.8 grams of water.v 95.7 grams of this mixture was added to each of the 100-gram samples of the above emulsion to give four coating formulations each having a different range of particle size of the dispersed phase. Each of the four coatings was applied to paper as a 4 mil wet thickness coating. Drying was accomplished as in Example 1 and the samples were conditioned at 50% relative humidity. The coated samples were charged as in Example 1 and then exposed to the step tablet as described. In the coating which contained air voids in the range of 34 microns an intensity difference was noted between steps 5 and.6; in the coating which contained air voids in the range of 0.5-2 microns an intensity difference was noted between steps 3 and 4; in the coating which contained air voids in the range of 0.51

micron an intensity difference was noted between steps 2 and 3; and in the coating which contained air voids in a range less than 0.5 micron an intensity difference was noted between steps 1 and 2.

Example 6 This example demonstrates the use of an organic filmforming photoconductor. Since the photoconductor poly- (N-vinyl carbazole) is soluble in benzene, the coating was prepared as a water-in-oil emulsion, the poly(N-vinyl carbazole) solution being the continuous phase.

To 50 grams of a 20% dispersion of poly(N-vinyl carbazole) in benzene was added 58 grams of benzene, 1.3 grams of Triton X400 (stearyl dimethyl benzyl ammonium chloride) and 0.7 gram of Arlacel C (sorbitan sesquioleate). This mixture was stirred rapidly while 30 grams of water was added dropwise to form the water-in-oil emulsion. As a control sample, 58 grams of benzene, 1.3 grams of Triton X-400, and 0.7 grams of Arlacel C were added to 50 grams of a 20% dispersion of poly(N-vinyl carbazole) in benzene. The two coatings were then applied to paper by a No. 36 wire-wound rod. Drying was accomplished at room temperature for 10 minutes, or until the coatings had sufficient gel strength so that the emulsion would not break. The samples were then further dried by heating in an air-circulating oven at 260 F. for about two minutes. The samples were charged as in Example 1 and exposed to an ultraviolet lamp (peak wavelength 2660 A.) through a step tablet for 15 seconds. The coating which contained the air voids was completely discharged by about 6% of the total incident radiation, while the coating without the air voids was not completely discharged by 100% of the incident radiation. An intensity difference was noted between steps 10 and 11 for the coating containing air voids; while in the control, which was a solid film the intensity difference was noted between steps and 6.

Although it is not known precisely why the incorporation of voids into a photoconductive film brings about the marked and unexpected increase in sensitivity shown in the examples, the following is offered as a possible explanation.

The binder together with the dark adapted photoconductive material has a dielectric constant which may be denoted by se The air in the voids in the film has a dielectric constant 6 which is materially lower than that of the coating material. The maximum voltage gradient attainable at the surface of the coating is probably characteristic of the coating material and independent of whether or not the coating contains voids. It might be expected then that maximum voltage to which a surface could be charged would not be affected by bubble content in the coating (but would depend on the coating thickness which We assume to be held constant). However, it appears that the introduction of voids in the coating reduces the dielectric constant of the coating with the result that a lesser surface charge is required to attain this maximum surface potential. The discharge of the bubbled coating apparently thus involves a lesser flow of charge and the surface is correspondingly more sensitive to light, or is faster in the photographic sense. This is consistent, in a qualitative way at least, with the experimental findings.

The decrease in dielectric constant will not, however, be directly proportional to the voids introduced because of the tendency for the electrostatic lines to concentrate in the region of higher dielectric constant. On the other hand, the volume charge is probably directly proportional to the density of the coating layer and is thus reduced in direct proportion to the voids introduced. Thus it may be inferred that the maximum attainable surface charge will be reduced by a lesser factor than will the volume charge density. The result of this is an extension of the range of linear response for a bubble-coated sheet beyond that found for bubble-free coatings. In this possible mechanism which if offered the theoretical implica- 10 tions of possible charge accumulations at the free surfaces between bubbles and coating material has not been considered.

There are, however, limits to the void sizes which bring about these improved performance characteristics. Thus any substantial numbers of voids larger than about ten microns are not desirable since apparently the presence of voids of this size contributes little to'improving the performance of the resulting photoconductive film. -Data in Example 5 show that any substantial number of voids less than 0.5 micron is also to be avoided, thus giving a preferred void size range from between about 0.5 and 10 microns. In addition to being characterized in terms of void size range, the photoconductive film of this invention may be characterized by its density which should range between about 0.10 and 0.6 gm./cc. without the particulate photoconductive material: e.g., between about 0.3 and 2.5 gm./cc. with ZnO photoconductive material. Thus density in fact limits the number of voids, or conversely, the amount of binder present, in any unit volume of film and is to some extent dependent upon the dielectric constant of the binder material used, whether it serves in the role as a means for supporting the photoconductive particles or as the photoconductive material itself.

The void size may be controlled in several ways. In the preferred method of forming the photoconductive film of this invention by the use of an emulsion coating, void size is controlled by the size of the liquid globules forming the discontinuous phase. Many techniques are known in the art for this type of control, e.g., the use of a homogenizer (such as a Manton-Gaulin Homogenizer).

The thickness of the finally dried photoconductive film may vary between about 0.1 and 2 mils. The actual thickness will depend upon the density of the coating and upon practical limitations associated with the apparatus used to coat the liquid on the substrates and the end use of the product.

The binder materials used to suspend photoconductive particles may be either of a thermosetting nature (e.g., casein or alpha protein) or of the thermoplastic type. Among the thermoplastics may be listed the well known film forming materials such as polyvinyls, polyacetates and their copolymers, polyolefins (e.g., polyethylene), polymers of the acrylic acids and their various derivatives, polystyrene, and elastomers including natural and synthetic rubbers. Mixtures of binders, casein and hydrolyzed starch for example, are also, of course, suitable. It will be seen that the film of this invention makes it possible to use a much greater variety of binder materials than heretofore possible.

Many of the inorganic photoconductive materials have been identified above. In general they may be defined as the oxides, sulfides, selenides, tellurides, and iodides of cadmium, mercury, antimony, bismuth, thallium, indium, molybdenum, aluminum, lead and zinc. In addition, mixtures of these as well as arsenic trisulfide, cadmium arsenide, lead chromate and selenium should be mentioned. The photoconductive particles preferably have a high resistivity when dark adapted. The spectral response of the film will depend upon the choice of photoconductive particles used; and as noted above any dyes used will be chosen to complement the spectral response.

These photoconductive materials are generally finely divided solid particles which tend to agglomerate when mixed into the liquid coating combination. This agglomeration, which is apparently desirable, means that ultimate particle size is relatively unimportant. Generally, the sensitivity of the coating decreases as the size of the photoconductor agglomerates decreases. In formulating the coating it is desirable to disperse the photoconductive particle agglomerates only enough to give a smooth coating on the substrate.

Because the photoconductive particles in the film described herein are suspended in a binder also containing voids some consideration must be given to the weight ratio of photoconductive particles to binder material. The minimum amount of photoconductive particles is largely determined by the performance desired from the .photo conductive film and can generally be defined as 4 parts by weight to one part of binder. The upper limit is primarily determined by the quantity of photoconductive particles which can be held within the binder and it may generally be defined as about 8 to 10 parts by weight to one part of binder.

The substrate may be either an insuating material such as paper, or an electrically conducting material such as metal foil or paper loaded with carbon black. It preferably has a lower resistivity than the film attached thereto. The

substrate may be flexible, semi-flexible or rigid, depending.

on its ultimate use. For office copying it will normally be a thin paper.

It will be seen from the above examples that the voidcontaining photosensitive coating of this invention is considerably more responsive to light exposure than an equal weight of the same coating without voids. This means that using lighter coating weights the coating described herein achieves a response equivalent to those now in use. These lighter coating weights in turn offer the possibility of using lighter-weight supporting substrates, a fact which con tributes to the overall decrease in weight. Moreover, the coating of this invention offers the possibility of using a wider rangeof binders than vheretofore possible and of forming photoconductive coatings which are pressure sensitive and moisture insensitive.

It will thus be seen that the objects set forth above, among those made apparentfrom the preceding description, are efficiently attained and, since certain changes may bemade in carrying out the above process, in the described product and in the constructions set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. Method of forming a printing base suitable for electrostatic. printing, comprising the steps of (a) dissolving a film-forming photoconductive material in a solvent having sufiicient volatility to permit its subsequent removal by evaporation fromthe resulting solution;

(b) introducing into said solution a volatile liquid immiscible with said solvent thereby to form an emulsion in which said solution of said film-forming photoconductive material is the continuous phase;

(c) applying a film of said emulsion to thesurface of a substrate thereby to deposit a coating on said surface; and

(d) drying said coating while retaining the structure of said emulsion to remove said solvent and said volatile liquid thereby to form a photoconductive coating characterized as being a substantially continuous film containing air voids essentially corresponding in size and location .to the globules of the discontinuous phase of said emulsion, substantially all of said air voids being at least 0.5 micron in size.

2. Method in accordance with claim 1 wherein said film-forming photoconductive material is a mixture of solid particles of a photoconductive material and a non photoconductive binder.

3. Method in accordance with claim 1 wherein said film-forming photoconductive material is an organic material, said volatile liquid is water and said emulsion is the water-in-oil type,

12 4. Method in accordance with claim lturther char acterized by the step of adding to said solutiona dye capable of widening the spectral band to which saidv photoconductive material is responsive.

5. Method of forming a printing base suitable for electrostatic printing, comprising the steps of (a) dissolving a film-forming binder material in Water to form a solution;

(b) incorporating into said solution solid particles of a photoconductive material;

(c) introducing into said solution a volatile organic water-immiscible liquid having a boiling point above that of water thereby to form an oil-in-water emulsion, the globules of the discontinuous phase being at least 0.5 micron in size;

((1) applying a film of said emulsion to the surface of a substrate thereby to deposit a coating on said surface; and

(e) drying said coating while retaining the structure of said emulsion to remove first a portion of said water to set said binder material into a matrix and then drying further to remove the remaining water and said volatile organic liquid thereby to form a photoconductive coating characterized as being a substantially continuous film of said binder contain-, ing throughout said photoconductive material and multitudinous air-binder interfaces essentially corresponding in size and location to-said globules forming said discontinuous phase of the original emulsion.

6. Method in accordance with claim .5 wherein said binder material is casein.

7. Method in accordance with claim 5 wherein said volatile organic liquid is kerosene.

8. Method in accordance with claim 5 wherein said photoconductive material is zinc oxide.

9. Method of forming a printing base suitable for electrostatic printing comprising the steps of (a) dissolving a water-insoluble film-forming binder material in a Water-immiscible volatile organic solvent;

(b) incorporating into the resulting solution solid par? ticles of a photoconductive material;

(c) forming with said solution a water-in-oil emulsion, the water globules of the discontinuous phase being at least 0.5 micron in size;

((1) applying a film of said emulsion to the surface of a substrate thereby to deposit a coating on said surface; and

(e) drying said coating to remove said organic solvent.

and said water and to form a photoconductive coating characterized as being a substantially continuous film of said binder containing throughout said photoconductive material .and air voids essentially corresponding in size and location to said water globules of the original emulsion. 10. Method in accordance with claim 9 wherein said film-forming binder material is ethyl cellulose and said volatile organic solvent is benzene.

References Cited UNITED STATES PATENTS 3,108,009 10/1963 Clancy et al. 11746 3,255,039 6/1966 Dalton 117-215 ALFRED L. LEAVITT, Primary Examiner.

W. L. JARVIS, Assistant Examiner. 

1. METHOD OF FORMING A PRINTING BASE SUITABLE FOR ELECTROSTATIC PRINTING, COMPRISING THE STEPS OF (A) DISSOLVING A FILM-FORMING PHOTOCONDUCTIVE MATERIAL IN A SOLVENT HAVING SUFFICIENT VOLATILITY TO PERMIT ITS SUBSEQUENT REMOVAL BY EVAPORATION FROM THE RESULTING SOLUTION; (B) INTRODUCING INTO SAID SOLUTION A VOLATILE LIQUID IMMISCIBLE WITH SAID SOLVENT THEREBY TO FORM AN EMULSION IN WHICH SAID SOLUTION OF SAID FILM-FORMING PHOTOCONDUCTIVE MATERIAL IS THE CONTINUOUS PHASE; (C) APPLYING A FILM OF SAID EMULSION TO THE SURFACE OF A SUBSTRATE THEREBY T DEPOSIT A COATING ON SAID SURFACE; AND 